In the domestic field it is known to use electric ovens for preparing and cooking food. Such electric ovens generally comprise a box-like body with an opening and closing door, an internal base to support the food to be prepared, and heating elements, for example electric resistances controlled by relays or TRIAC. Such electric heating elements can be positioned both on the high part of the oven and also on the lower part, or laterally.
The ovens are provided with a control unit to manage the cooking times and temperatures, according to the type of cooking chosen by the user by means of a suitable interface.
To control the temperature of the oven, a sensor to detect the temperature is usually provided, located on an internal wall of the oven. An example of an oven provided with a control system that includes a temperature sensor within the cooking cavity is disclosed in US 2007/084849.
A first limit of known electric ovens is precisely the position of the temperature sensor, since the temperature that one wants to measure and estimate inside an oven is the one in the central zone of the oven, where the food is actually cooked or prepared, while the detection sensor is situated in the lateral wall. In short therefore, the desired temperature set by the user according to the type of cooking required, which is also the one that should be kept constantly controlled, is different from the temperature actually measured.
Another limit of known electric ovens lies in the way they control and regulate the internal temperature. In fact, known electric ovens usually have an ON-OFF drive with regard to the power to be supplied to the electric heating elements and the temperature is regulated by mechanical or electronic hysteresis systems.
In mechanical hysteresis regulation, two temperature thresholds are set, a maximum and a minimum, therefore the temperature sensor located on the lateral wall of the oven measures a certain temperature which is compared with the two thresholds. The oven supplies maximum power until a temperature value is reached, usually higher than the one set as maximum, and then switches off. Then, when a temperature lower than the desired temperature is detected, the oven switches itself on again to increase the temperature to the desired value. The power is therefore sized for rapid heating and not for maintaining a constant temperature.
In electronic hysteresis regulation, a maximum temperature threshold and a minimum temperature threshold are also set in the oven, and in this case the electronic regulation part allows to reduce the minimum difference between the two thresholds.
In both regulation systems, mechanical or electronic hysteresis, there are in any case problems relating to delays in measuring and intervening of the system to regulate the temperature, gross errors in evaluating the temperature and often too high powers are supplied.
In both types of functioning therefore, whether electronic or mechanical hysteresis, there are unwanted oscillations in the developments of the temperature over time, and hence of the temperature desired at the center of the oven. For example in the case of mechanical hysteresis, a difference in temperature of about 10° C. on the temperature sensor located on the wall of the cooking chamber leads to a difference of about 40° C. at the center of the oven. As we said, in the case of electronic hysteresis this difference is reduced, but it is still considerable.
Another problem of known electric ovens is that it is difficult to keep the temperature in the cooking chamber substantially constant and at a predetermined value; this makes it very complicated to cook food at relatively low temperatures, for example below 100° C., without running the risk of degrading the food or of obtaining an approximate cooking. In fact, one should remember that, in known ovens, the lower the operating temperature, the greater the oscillations in mean temperature.
WO 2013/063691 discloses a method of controlling a heat-generation element to control a temperature within a space, which provides to use a sensor for detecting an initial temperature, to supply electric current at a maximum output power for a period of time, to detect a second temperature at the end of the period of time, to determine a first temperature difference between the initial temperature and the second temperature, to determine a maximum error between a sensed temperature at a selected time and an ambient temperature based on a pre-determined relationship between the first temperature difference and the maximum error.
However, the solution proposed in this document does not solve the above-mentioned problems, and particularly those related to the unwanted oscillations in the developments of the temperature over the time.
There is therefore a need to perfect an electric apparatus for cooking and/or heating food, and a corresponding temperature control method and device for said electric apparatus, which can overcome at least one of the disadvantages of the state of the art.
In particular, one purpose of the present invention is to make available an electric apparatus for cooking and/or heating food, and corresponding temperature control method and device for said electric apparatus which, although using a sensor for measuring the temperature which is positioned in a wall of the cooking chamber of the apparatus, is able to estimate and maintain a substantially constant temperature at the center of the cooking chamber, that is, in the zone where the user really wants to control and maintain a set temperature value.
Another purpose of the present invention is also to perfect a method to control the temperature of electric apparatuses for cooking and/or heating food which allows to optimize energy consumption of the electric apparatus and which allows to optimize the power supply steps also according to dynamic parameters and possibly also the construction parameters of the electric apparatus to be controlled.
Another purpose of the present invention is to perfect a temperature control method which allows to cook or prepare food in an electric apparatus even at low temperature both efficiently and completely, without any risk of causing damage to said food, for example for preparing yoghurt, for leavening, for cooking meat at low temperature, for cooking without losing water and others.
Another purpose of the present invention is to obtain a device for actuating the temperature control method in electric ovens.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.